await population of a stream

In my MVC controller, i have an action that will populate a stream which i will use later:

[HttpPost]
public async Task<HttpResponseMessage> BuildFileContent(FileParameters fileParams)
{
    byte[] output;
    if (fileParams != null)
    {
        using (var stream = new MemoryStream())
        {
            await Task.Run(() => PopulateFile(fileParams, stream)); // Here i populate the stream
            stream.Flush();
            output = stream.ToArray();
        }
        var guid = Guid.NewGuid().ToString();
        FileContents.Add(guid, output); // just save the content for later use
        return this.Request.CreateResponse(HttpStatusCode.OK, new { Value = guid });
    }

    return this.Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Invalid parameters");
}

I had this problem because ReSharper is giving out a warning: access on disposed closure on the line PopulateFile(fileParams, stream) and pointing out stream.

I have searched for the meaning of it and it seems that it warns that i could possibly use an already disposed stream.

I want to await the population of the stream because the task could take long.

Is there a way that I can wait the population of the stream without ReSharper giving this warning? Moreover, i am calling stream inside the using clause, the stream should not yet be disposed until i call flush or the last line of the using clause is executed right? So why does ReSharper warn about accessing a disposed stream?